Implantable stimulation devices are devices that generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of metallic material such as titanium for example. The case 12 typically holds the circuitry and battery 14 (FIG. 2B) necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 10 is coupled to electrodes 16 via one or more electrode leads (two such leads 18 are shown), such that the electrodes 16 form an electrode array 20. The electrodes 16 are carried on a flexible body 22, which also houses the individual signal wires 24 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on each lead, although the number of leads and electrodes is application specific and therefore can vary. The leads 18 couple to the IPG 10 using lead connectors 26, which are fixed in a header 28 comprising epoxy for example, which header is affixed to the case 12. In a SCS application, distal ends of electrode leads 18 are typically implanted on the right and left side of the dura within the patient's spinal cord. The proximal ends of leads 18 are then tunneled through the patient's tissue 100 to a distant location such as the buttocks where the IPG 10 is implanted, where the proximal leads ends are then connected to the lead connectors 26.
As shown in cross section in FIG. 2B, the IPG 10 typically includes an electronic substrate assembly including a printed circuit board (PCB) 30 containing various electronic components 32 necessary for operation of the IPG 10, some of which are described subsequently. Two coils are generally present in the IPG 10: a telemetry coil 34 used to transmit/receive data to/from an external controller 50 (FIG. 2A); and a charging coil 36 for charging or recharging the IPG's battery 14 using an external charger 70 (FIG. 4A). These coils 34 and 36 are also shown in the perspective view of the IPG 10 in FIG. 1B, which omits the case 12 for easier viewing. Although shown as inside in the case 12 in the Figures, the telemetry coil 34 can alternatively be fixed in header 28. Coils 34 and 36 may alternative be combined into a single telemetry/charging coil.
FIG. 2A shows plan views of the external controller 50, and FIG. 2B shows it in cross section and in relation to the IPG 10 during a communication session. The external controller 50, such as a hand-held portable programmer or a clinician's programmer, is used to set or adjust the therapy settings the IPG 10 will provide to the patient (such as which electrodes 16 are active, whether such electrodes sink and source current, and the duration, frequency, and amplitude of pulses formed at the electrodes, etc.). The external controller 50 can also act as a receiver of data from the IPG 10, such as various data reporting on the IPG's status, the level of the IPG 10's battery 14, and other parameters measured or logged at the IPG 10. Such communications can occur bi-directionally via link 75.
As shown in FIG. 2B, the external controller 50 contains a PCB 51 on which electronic components 52 are placed to control operation of the external controller 50. The external controller 50 is powered by a battery 53, but could also be powered by plugging it into a wall outlet for example. A telemetry coil 54 is also present in the external controller 50, which will be discussed further below. A case 59, typically made of plastic, houses the internal components of the external controller 50. The external controller 50 typically comprises a user interface 55 similar to that used for a portable computer, cell phone, or other hand held electronic device, including touchable buttons 56 and a display 57. A port 58 allows the external controller to be electrically coupled to a power source, to other computer devices, etc.
Wireless data transfer between the external controller 50 and the IPG 10 via link 75 takes place via magnetic inductive coupling between coils 54 and 34, either of which can act as the transmitter or the receiver to enable two-way communication between the two devices. Referring to FIG. 3, which depicts circuitry in these devices, when a series of digital data bits (FSK data 47) is to be sent from the external controller 50 to the IPG 10, control circuitry 60 (e.g., a microcontroller) provides these bits in sequence to a modulator 61. Modulator 61 energizes coil 54 with an alternating current (AC) whose frequency is modulated in accordance with the state of the data bit currently being transferred in accordance with a Frequency Shift Keying (FSK) protocol. For example, the coil 54 may nominally be tuned to resonate at 125 kHz in accordance with the inductance of the coil 54 and a tuning capacitor (not shown), which data states ‘0’ and ‘1’ altering this center frequency to f0=121 kHz and f1=129 kHz respectively. The frequency-modulated current through the coil 54 in turn generates a frequency-modulated magnetic field comprising link 75, which in turn induces a frequency-modulated current in the IPG's telemetry coil 34. This received signal is demodulated 43 back into the series of digital data bits, and sent to control circuitry 38 (e.g., a microcontroller) in the IPG 10 for interpretation. Data telemetry in the opposite direction from IPG 10 to external controller 50 occurs similarly via modulator 41 and demodulator 62. Inductive coupling via link 75 occurs transcutaneously, i.e., through the patient's tissue 100.
Other means for electro-magnetically communicating between the external controller 50 and IPG 10 via link 75 are known as well, including RF communications such as Bluetooth, Zigbee, etc., that are enabled patch, wire, or slot antennas. In this instance, link 75 would comprise a longer-range electromagnetic field, rather than the near-field magnetic field enabled by coils 54 and 34.
FIG. 4A shows a plan view of the external charger 70, and FIG. 4B shows it in cross section and in relation to the IPG 10 during a charging session. The external charger 70 is used to wirelessly charge (or recharge) the IPG's battery 14, and includes at least one PCB 72 (two are shown; see U.S. Patent Application Publication 2008/0027500); electronic components 74, some of which are subsequently discussed; a charging coil 76; and a battery 78 for providing operational power for the external charger 70 and for the production of a magnetic charging field 80 from the coil 76. These components are typically housed within a case 77, which may be made of plastic for example.
The external charger 70 has a user interface 82, which typically comprises an on/off switch 84 to activate the production of the magnetic charging field 80; an LED 86 to indicate the status of the on/off switch 84; and a speaker 88. The speaker 88 emits a “beep” for example if the external charger 70 detects that its charging coil 76 is not in good alignment with the charging coil 36 in the IPG 10 during a charging session, as discussed further below. The external charger 70 is sized to be hand held and portable, and may be placed in a pouch around a patient's waist to position the external charger 70 in alignment with the IPG 10 during a charging session. Typically, the external charger 70 is touching the patient's tissue 100 during a charging session as shown, although the patient's clothing or the material of the pouch may intervene.
Wireless power transfer from the external charger 70 to the IPG 10 occurs by magnetic inductive coupling between coils 76 and 36. Referring to FIG. 5, when the external charger 70 is activated (e.g., on/off switch 84 is pressed), a charging circuit 94 under control of control circuitry 92 (e.g., a microcontroller) energizes coil 76 with a non-data-modulated AC current (Icharge) to create the magnetic charging field 80. The frequency of the magnetic charging field may be on the order of 80 kHz for example, and may be set by the inductance of the coil 76 and the capacitance of a tuning capacitor (not shown). The magnetic charging field 80 induces a current in the IPG 10's charging coil 36, which current is rectified 44 to DC levels and used to provide a charging current (Ibat) to recharge the IPG's battery 14, perhaps under the control of charging and battery protection circuitry 46 as shown. This again occurs transcutaneously.
The IPG 10 can also communicate data back to the external charger 70 using Load Shift Keying (LSK) telemetry. Relevant data, such as the capacity of the battery, is sent from control circuitry 38 in the IPG 10 to a LSK modulator 40, which creates a series of digital data bits (LSK data 48). This data is input to the gate of a load transistor 42 to modulate the impedance of the charging coil 36 in the IPG 10. Such modulation of the charging coil 36 is detectable at the external charger 70 due to the mutual inductance between the coils 76 and 36, and will change the magnitude of the AC voltage needed at coil 76 (Vcoil) to drive the charging current, Icharge. If coil 36 is shorted (LSK data=1), Vcoil increases (Vcoil1) to maintain Icharge; if not shorted (LSK data=0), Vcoil decreases (Vcoil0), as shown in the waveform in FIG. 5. LSK demodulator 96 in the external charger 70 can detect these changes in Vcoil (ΔV) to recover the series of digital data bits, which data is then received at control circuitry 92 so that appropriate action can be taken, such as ceasing production of the magnetic charging field 80 (i.e., setting Icharge to zero) because the battery 14 in the IPG 10 is full. Note that the nature of LSK telemetry as described here only allows for telemetry from the IPG 10 to the external charger 70 when a magnetic charging field 80 is being produced. See, e.g., U.S. Patent Application Publication 2013/0123881 for further details regarding the use of LSK telemetry in an external charger system.
The inventor is concerned about certain problems with traditional means of wireless communications between an external controller 50 and the IPG 10, and with traditional means of charging an IPG 10 using an external charger 70. The inventor's concerns regarding communications are discussed first.
As is known, wireless communications to and from the IPG 10 can be attenuated by the conductive material of the case 12 as well as other conductive structures present in the IPG 10 and the external controller 50. Especially when magnetic induction is used as the means for establishing communication link 75 for example, the generated AC magnetic fields will create eddy currents in such conductive structures, which essentially act as an unwanted sink for the energy in the field, thus reducing the distance at which communications and charging can reliably occur. See, e.g., U.S. Pat. No. 8,457,756.
Previous IPGs 10 have used non-conductive ceramic materials for the case 12, see, e.g., U.S. Pat. No. 7,351,921, which would reduce attenuation of wireless communications in IPGs using internal coils. However, ceramic materials are also brittle and difficult to work with. Ceramic case components further require brazing to mechanically couple them together or to other metallic components, which can be difficult to perform.
Previous approaches have used optical radiation instead of electromagnetic fields as the means to communicate with an implantable medical device. For example, U.S. Pat. No. 5,556,421 discloses a pacemaker which has photoemitter such as a Light Emitting Diode (LED), and a photodetector such as a phototransistor, for respectively transmitting data to and receiving data from a device external to the patient. See FIG. 15 of the '421 patent. However, in the '421 patent, the photoemitter and photodetector are contained within the header of the pacemaker, similar to the header 28 for the IPG 10 described earlier (FIG. 1A). The header is described in the '421 patent as suitably translucent to the wavelengths of optical radiation at which the LED and photodiode operate (within the range of 640 to 940 nm).
The inventor however finds the optical communication approach of the '421 patent to be problematic, in particular because the optical elements are contained within the header of the implantable medical device. The three-dimensional shape of the header makes optical transmission and reception difficult, as optical radiation will reflect at the outer surfaces of the header and other reflective components in the header, such as the lead connectors 26 (see, e.g., FIG. 1A). Optical radiation will also refract, attenuate, and disperse in the header material. Additionally, there may be little room in the header to accommodate optical elements. This is particularly problematic in a SCS IPG, which comprises many electrodes (e.g., 16 or 32), and hence requires long lead connectors 26, or more lead connectors, in the header 28. Providing optical elements in the header provides further concerns that additional feedthrough pins between the header and the interior of the case would be necessary, complicating IPG design and potentially impacting reliability.
U.S. Pat. No. 6,243,608 also discloses a pacemaker that can communicate optically with an external device, although once again in this reference, the optical element is contained in the header, thus suffering from the same problems discussed above with reference to the '421 patent. (Specifically, this pacemaker has only a photoemitter and thus can only communicate optically with the external device in one direction; communication from the external device to the pacemaker occurs via magnetic induction between two coils). In the text associated with one embodiment, see FIG. 6 of the '608 patent, it is mentioned that the photoemitter can be located in an electronics module inside the pacemaker case. But in this instance, the photoemitter transmits light from inside the case to the translucent header. This too is not practical. Although not discussed in detail in the '608 patent, this approach requires porting the optical radiation through the feedthrough between the case and header in some fashion, which would attenuate the radiation, and complicate feedthrough design. It is noted that a mirror may need to be provided in the header to direct the optical radiation to the external device, or that a portion of the outer surface of the header be shaped as a lens, both of which are complicated, expensive, and could be expected to attenuate the radiation.
U.S. Pat. No. 7,447,533 discloses a pacemaker in which a photoemitter and photodetector are used to detect a physiological parameter, such as blood flow (photoplethysmography). In one example, see FIGS. 7 and 8 of the '533 patent, an aperture is formed in one of the flat sides of the case that accommodates an assembly containing the optical elements. Once positioned in place, the assembly is welded to the case. Nonetheless, the '533 patent is not relevant to the inventor's concern regarding communications between an implant and an external device. The optical elements in the '533 are not used to send and receive a series of data bits, and are not used to communicate optical radiation externally to the implant. Instead, the photoemitter provides radiation that reflects off the patient's tissue, which reflection is detected at the implant's photodetector to determine the physiological parameter. (If a dual-wavelength photoemitter is used, the wavelengths are enabled in an alternating fashion). For communications between the implant and the external device, the '533 patent instead uses an electromagnetic antenna operable with radio waves (e.g., 10-15 MHz). U.S. Pat. No. 5,902,326 is similar, although in this patent the optical elements are used to detect a different physiological parameter, namely blood oxygen content (oximetry).
U.S. Patent Application Publication 2009/0076353 also comprises a pacemaker having an aperture on one of the flat sides of the case that accommodates an optical sensor assembly, which again can be welded to the case. However, the unique particulars of the '353 Publication render it unsuitable for data communication external to the implant. The optical sensor assembly is designed to detect yet another physiological parameter, in this case analytes such as Potassium ions. As described in the '353 Publication, such analytes are designed to diffuse through the optical sensor assembly where they meet with a chemical sensing element. Photoemitters in the assembly are made to reflect off of this chemical sensing element. The chemical sensing element's optical properties change in the presence of the analyte, and so reflections are received at a photodetector in the optical sensor assembly to measure the analyte. Indeed, the unique particulars of this publication render it unsuitable for external data communications, as an overlying cover layer is included to block ambient light from entering the optical sensor assembly, and also to prevent the light from the photoemitters from escaping the optical sensor assembly.
The inventor is also concerned about shortcomings concerning charging an implantable medical device battery. In particular, the inventor is concerned that charging is hampered by difficulty in determining the alignment between the external charger 70 and the IPG 10.
It is generally desirable to charge the IPG's battery 14 as quickly as possible to minimize inconvenience to the patient. One way to decrease charging time is to increase the strength of the magnetic charging field 80 by increasing Icharge in the charging coil 76 of the external charger 70. Increasing the magnetic charging field 80 will increase the current/voltage induced in the coil 36 of the IPG 10, which increases the battery charging current, Ibat, hence charging the battery 14 faster.
However, the strength of the magnetic charging field 80 can only be increased so far before heating becomes a concern. Heating is an inevitable side effect of inductive charging using magnetic fields, and can result because of activation of relevant charging circuitry in the external charger 70 or IPG 10, or as a result of eddy currents formed by the magnetic charging field 80 in conductive structures in either device. Heating is a safety concern. The external charger 70 is usually in contact with the patient's tissue 100 during a charging session, and of course the IPG 10 is inside the patient. If the temperature of either exceeds a given safe temperature, the patient's tissue may be aggravated or damaged.
The alignment between the external charger 70 and the IPG 10 can affect heating, as shown in FIGS. 6A and 6B. In FIG. 6A, the charging coils 76 and 36 in the external charger 70 and the IPG 10 are well aligned, because the axes 76′ and 36′ around which the coils 76 and 36 are wound are collinear. As such, these coils 76 and 36 are well coupled electrically, meaning that a higher percentage of the power expended at coil 76 in creating the magnetic charging field 80 is actually received at coil 36, which leads to higher values for Ibat. In FIG. 6B, the charging coils 76 and 36 are laterally misaligned (d), which reduces the electrical coupling between the coils. Increasing the vertical distance x between the coils 76 and 36 (FIG. 6C), or increasing the angle (θ) between the preferably parallel planes in which they reside (FIG. 6D), will also reduce coupling.
If it is desired that the alignment scenarios of FIGS. 6A and 6B charge the battery 14 at the same rate (Ibat=Y), then a higher value for Icharge (Icharge>X) will be needed in the misaligned scenario of FIG. 6B compared to the well-aligned scenario of FIG. 6A (Icharge=X). A higher value for Icharge in FIG. 6B will create a more intense magnetic charging field 80 that tend to increase the temperature of the environment (T>Z) when compared to the temperature of the environment in FIG. 6A (T=Z). If it is desired that the temperature be the same for both scenarios, then Icharge can be lowered in FIG. 6B, but this will also lower Ibat, and hence the battery 14 in that scenario would take longer to charge. In short, misalignment between the external charger 70 and the IPG 10 is not desired.
Accordingly, the art has disclosed several manners for determining misalignment between an external charger and an IPG, which techniques usually result in some form of user-discernible output letting the patient know when alignment is poor (such as via speaker 88 discussed earlier). Such techniques may also inform a patient how to fix the alignment, such as by indicating a direction the external charger should be moved relative to the IPG 10. See, e.g., U.S. Pat. Nos. 8,473,066 and 8,311,638.
Previous external charger alignment techniques however are difficult to implement, and may not precisely determine alignment as they rely on inferences gleaned from electrical measurements taken during the charging session. For example, one prior art alignment techniques relies on determining the loading of the charging coil in the external charger during production of the magnetic charging field. Specifically, the voltage across the charging coil (Vcoil) is reviewed at the external charger and compared to a Vcoil threshold to determine alignment. This technique though suffers in its inability to distinguish between the scenarios of FIGS. 6B and 6C for example. In either of these scenarios, Vcoil would be higher due to poor coupling, but in FIG. 6B the poor coupling arises from misalignment, whereas in FIG. 6C the alignment is as good as it can be given the IPG 10's depth (x). A modification to this technique helpful in distinguishing these scenarios requires transmitting the magnetic charging field at different frequencies and measuring the input current to the charging coil in the external charger to estimate an implant depth (x), and thus to set an appropriate Vcoil threshold. See, e.g., U.S. Patent Application Publication 2010/0137948. However, the additional overhead of having to produce magnetic charging fields at different frequencies makes this technique complicated.
Other alignment techniques require the external charger to have positioning coils in addition to the main charging coil (e.g., 76). In these techniques, measurements taken from the positioning coils during the charging session are used to determine misalignment, and to indicate a direction the external charger can be moved to improve alignment (coupling). See, e.g., U.S. Pat. Nos. 8,473,066 and 8,311,638. The requirement of additional coils beyond the main charging coil though complicates the design of the external charger.
Still other alignment techniques employ electromagnetic (EM) telemetry from the IPG, see, e.g., U.S. Patent Application Publications 2013/0096651 and 2011/0087307, which adds complexity to both the IPG and the external charger. Moreover, EM telemetry may be difficult to employ while the external charger is generating a magnetic charging field (e.g., 80), because such field is relatively strong, and may add significant noise to the EM telemetry signal. Thus it may be necessary to periodically cease the production of the magnetic charging field during a charging session to allow such telemetry from the IPG to the external charger to occur, which inconveniently lengthens the duration of the charging session.
The inventor is further concerned that LSK telemetry is limited in its ability to communicate information from the IPG the external charger. First, as noted earlier, LSK telemetry is only useful when the external charger is producing a magnetic charging field, thus hampering the ability of the IPG to communicate with the external charger, prior to starting a charging session for example. Moreover, LSK telemetry may be difficult to demodulate (e.g., FIGS. 5, 96). Vcoil, the parameter assessed by LSK demodulator 100, can vary in magnitude as the alignment between the external charger and IPG varies during a charging session, which is typical. Likewise, ΔV, the difference in Vcoil for each of the logic states being transmitted by the IPG, can vary and may also be relatively small and hard to detect depending on the coupling.